There have been various different types and kinds of full-color display systems, such as for use with computer-driven projection display panels, computer monitor screens, and television screens. Inventive techniques for producing such a full-color display, are disclosed in the foregoing-mentioned parent patent applications. The techniques include the use of twisted nematic liquid crystal display panels, which are stacked or arranged in series along a common optical path. Each one of the panels, together with a set of interleaved polarizers, pass three different primary colors, such as red, blue and green. The three primary colors are selectively and additively combined to provide a group of full colors. In this regard, by selectively either energizing or deenergizing certain ones of the panels, combinations of colors can be achieved from the output stage of the display system.
According to the inventive techniques employed in the foregoing mentioned pending patent applications, each one of the three stages of the stacked panel system, can produce multiple numbers of shades of each one of the three primary colors. The patentable techniques, thus, enable a large number of combinations of colors to be passed by the display system.
The twisted nematic liquid crystal display panels, while being quite effective, are limited in their speed of operation. Thus, the more modern high-speed active matrix display panels have been developed. Such an active matrix panel is a single panel employing three different color elements for each pixel. Therefore, there is no need to have three separate panels to provide a multi-color display. The active matrix panel, employs pixel elements each having three color subpixel components, and operates at a much higher speed than its earlier twisted, nematic panel.
By employing three color components for each pixel, a color additive process, of selecting individual ones of the three colors, permits a total of eight colors, which may be passed for each pixel. However, in order to increase the number of colors from the pixel, various different shades or intensity levels for each color component is required. One approach to achieve such intensity levels would be to provide additional hardware circuits for driving the pixel elements through various different color intensity levels for each one of the three color components in a pixel element. While this approach may be satisfactory for some applications, such an approach would be quite expensive to manufacture.
In the foregoing mentioned patent applications, intensity levels have been produced by employing a duty-cycle modulation system. In this regard, each one of the three color element is either turned on or off, each raster display frame. However, when a color element is to be energized to produce a desired shade of a color, it will remain on for a certain average percentage of the time over a series of raster frames. The persistence of the eye of the viewer perceives a resulting color to be of a certain desired intensity or shade. In this approach, electronic control circuits are provided to cause the individual color pixel elements to be turned ON selectively, and OFF selectively, to achieve a desired color intensity level.
When employing such an electronically controlled modulation or duty cycle system in a high-speed display system, such as an active matrix display system, the difference of speed between the computer controlled modulation circuit, and the much higher speed display panel, causes an undesirable movement of the image. Such a movement of the image is undesirable and unwanted for certain applications.
In order to match the speed of the computer controlled circuit, as governed by the maximum speed of the computer driving it, to the much higher speed display panel, the use of memories, such as a bit map memory for storing an image to be displayed, can provide the necessary matching of the speeds. However, this technique is not entirely desirable for some applications, since there is a quantizing error introduced by employing such an approach. The resulting image is a digitized image, which is an approximation only of the desired analog signal. For more information relating to the problems of quantizing error, reference may be made to a book entitled "Digital Pictures Representation and Compression" by Arun N. Netravali and Barry G. Haskell.
The quantizing error causes an undesirable "contouring" or splotchiness of the resulting color images. In other words, when the number of shades or intensities of a given color is increased, due to the quantization of the picture image, the resulting color image is merely an approximation of a corresponding analog signal image of the color to be displayed, and the resulting image produced, is not entirely satisfactory.
In order to reduce the quantizing error to such a level that the contouring or splotchiness is reduced to an acceptable level, at least eight or nine bits of color information are required for a given color intensity. However, such a bit map memory required for a high speed display, would be excessively large in size, and unduly expensive, in order to match the speeds of the computer with the high-speed display. Additionally, should the modulation or duty cycle technique be employed to increase the number of possible color intensity levels, the duty cycle control circuits would be excessively complex, and thus be too expensive to manufacture for some applications.
Therefore, it would be highly desirable to produce a very large number of full colors from a high-speed display, without causing an undesirable movement of the resulting image caused by improper matching of speeds between the computer and the display, and also without contouring or splotchiness caused by excessive quantizing error associated with approximations of digital pictures.